U.S. patent number 4,931,760 [Application Number 07/095,988] was granted by the patent office on 1990-06-05 for uniform magnetic field generator.
This patent grant is currently assigned to Asahi Kasei Kogyo Kabushiki Kaisha. Invention is credited to Junji Yamaguchi, Terumasa Yamasaki.
United States Patent |
4,931,760 |
Yamaguchi , et al. |
June 5, 1990 |
Uniform magnetic field generator
Abstract
Magnetic field generators used in magnetic resonance imaging
instruments (MRI) need an extremely uniform and wide magnetic field
space. With the annulus of a field-generating annular magnet
composed of a plurality of permanent magnet blocks arranged
annularly, non-magnetic hold plates may be arranged in
correspondence with each of the permanent magnet blocks, and
field-regulating permanent magnet pieces may be fixed at stated
positions of the hold plated, thereby obtaining a uniform magnetic
field space.
Inventors: |
Yamaguchi; Junji (Shimoogino,
JP), Yamasaki; Terumasa (Samejima, JP) |
Assignee: |
Asahi Kasei Kogyo Kabushiki
Kaisha (Osaka, JP)
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Family
ID: |
17025726 |
Appl.
No.: |
07/095,988 |
Filed: |
September 14, 1987 |
Foreign Application Priority Data
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Oct 8, 1986 [JP] |
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61-238136 |
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Current U.S.
Class: |
335/306; 324/319;
335/299 |
Current CPC
Class: |
G01R
33/383 (20130101); G01R 33/3873 (20130101); H01F
7/02 (20130101) |
Current International
Class: |
G01R
33/383 (20060101); G01R 33/38 (20060101); G01R
33/3873 (20060101); H01F 7/02 (20060101); H01F
007/02 () |
Field of
Search: |
;335/306,299,296
;324/319,320 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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60-88407 |
|
May 1985 |
|
JP |
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61-88210 |
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Jun 1986 |
|
JP |
|
Other References
PCT Publication WO 84/00611 International App. No.: PCT/US83/01175
filed: Aug. 2, 1983 William Oldendorf inventor..
|
Primary Examiner: Broome; H.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas
Claims
What is claimed is:
1. A uniform magnetic field generator comprising:
means for generating a static magnetic field; and
means for regulating a uniformity of said static magnetic
field;
said generating means being an annular permanent magnet comprising
a plurality of permanent magnet blocks arranged annularly;
said regulating means comprising a plurality of adjustably
positioned permanent magnet pieces, said plurality of adjustably
positioned permanent magnet pieces positionable relative to said
generating means in said static magnetic field for regulating the
uniformity of said magnetic field, said regulating means further
comprising a non-magnetic hold plate for holding said plurality of
adustably positioned permanent magnetic pieces in said static
magnetic field.
2. A uniform magnetic field generator as claimed in claim 1,
wherein the residual magnetization of each of said permanent magnet
pieces is substantially identical to each other.
3. A uniform magnetic field generator as claimed in claim 2,
wherein said adjustably positioned permanent magnet pieces are of
different sizes.
4. A uniform magnetic field generator as claimed in claim 1,
wherein the size of each of said permanent magnet pieces is
substantially identical to each other.
5. A uniform magnetic field generator as claimed in claim 4,
wherein the residual magnetization of said adjustably positioned
permanent magnet pieces is different.
6. A uniform magnetic field generator as claimed in claim 1,
wherein said adjustably positioned permanent magnet pieces are
fitted to and removed from the non-magnetic hold plate.
7. A uniform magnetic field generator as claimed in claim 1,
wherein said permanent magnet blocks are continuous bodies and have
at least three different sectional areas in a section vertical to
the axial direction of said annulus within the same block.
8. A uniform magnetic field generator as claimed in claim 7,
wherein said plurality of permanent magnet blocks are of a
face-symmetrical shape.
9. A uniform magnetic field generator comprising:
means for generating a static magnetic field;
at least one permanent magnet piece for regulating the uniformity
of said static magnetic field; and means for holding said at least
one permanent magnet piece next to said means for generating said
static magnetic field;
wherein said means for generating a static magnetic field is a
permanent magnet, said permanent magnet is an annular magnet
comprising a plurality of permanent magnet blocks arranged
annularly, said magnet permanent magnet blocks are continuous
bodies and have at least three different sectional areas in a
section vertical to the axial direction of said annulus within the
same block, said permanent magnet blocks are of a face-symmetrical
shape, and wherein said selection of said permanent magnet blocks
cut vertical to the axis of said annular magnet is greater at the
two end portions that at the center main portion, and between said
center main portion and said two end portions, said block has
narrow portions of less sectional area than that of said center
main portion and said two end portions.
10. A uniform magnetic field generator as claimed in claim 9,
wherein each of said permanent magnet blocks is movable in two
axial orthogonal directions crossing the axial direction of said
annular magnet, and said permanent magnet blocks are rotatable
around said two axial directions.
11. A uniform magnetic field generator comprising:
means for generating a static magnetic field;
adjustably positioned permanent magnet pieces, said adjustably
positioned permanent magnet pieces positionable relative to said
static field generating means for regulating the uniformity of said
static magnetic field;
holding members for holding said adjustably positioned permanent
magnet pieces at desired positions;
a tube for fixing said holding members; and
means for moving said tube and fixed holding members into and out
of said static magnetic field generating means.
12. The uniform magnetic field generator according to claim 11,
wherein said moving means comprises rollers provided on said
holding members, and rail means for supporting said tube.
13. The uniform magnetic field generator according to claim 12,
further comprising a tube support for fixing said tube to said
static field generating means.
14. The uniform magnetic field generator according to claim 13,
further comprising rotator means for lifting up and rotating said
tube at said rail means.
15. A uniform magnetic field generator as claimed in claim 1,
wherein said plurality of adjustably positioned permanent magnetic
pieces are manually adjustable.
16. A uniform magnetic field generator as claimed in claim 1,
wherein said regulating means further comprises a non-magnetic
seat, said plurality of permanent magnetic pieces being fixed to
said holding plate via said seat.
17. A uniform magnetic field generator as claimed in claim 16,
further comprising a plurality of non-magnetic screws, and wherein
said hold plate includes a plurality of first open mounting holes,
and wherein said non-magnetic seat includes a second open mounting
hole, said non-magnetic seat being fixed to said hold plate by
inserting said non-magnetic screw through one of said plurality of
first open mounting holes of said non-magnetic hold plate and
through the second open mounting hole of said non-magnetic seat.
Description
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a magnetic field generator, and
more particularly to a highly uniform magnetic field generator used
in magnetic resonance imaging instruments (MRI).
DESCRIPTION OF THE PRIOR ART
The magnets used to generate magnetic fields can be grouped into
following two types, electromagnet and permanent magnet. An
electromagnet utilizes a magnetic field generated by an electric
current flowing through a coil. In a resistive electromagnet the
coil is made of copper or aluminum while in a superconducting
magnet the coil is made of superconducting materials such as Nb-Ti,
Nb.sub.3 Sn or the like, and the coil is cooled by liquid
helium.
Three kinds of magnetic field generators are used in MRI, the
resistive magnet type, superconducting magnet type, and permanent
magnet type. The resistive magnet type utilizes an air-core coil or
an iron-core coil. The initial cost of this type is low, but
cooling instruments for keeping the coil cool are necessary.
The superconducting magnet type employs a superconducting coil and
a cryostat which keeps the superconducting coil at a very low
temperature (i.e., the temperature of liquid helium). This magnet
type is high priced and maintenance is troublesome because of the
use of liquid helium. However, by using this type of magnet, a
stable, very high magnetic field such as 0.5-2 tesla can be
obtained. As a result, a high signal to noise ratio can be
obtained. Furthermore, at a magnetic field higher than 1 tesla, it
becomes possible to measure the spectra of not only .sup.1 H but
also .sup.31 p, .sup.23 Na and so on, and therefore more
information from within a living body can be obtained.
The permanent magnet type has a permanent magnet for generating a
magnetic field and a supporter for supporting the permanent magnet.
Furthermore, a magnetic pole piece for maintaining the uniformity
of the magnetic field and a magnetic yoke for forming a magnetic
circuit may be provided. This permanent magnet type has a number of
advantages, which include no need for field-generating power, low
cost of maintenance, little field leakage, and small installation
space.
A variety of proposals have been made to obtain a uniform magnetic
field space by use of a permanent magnet. For example, the
specifications of U.S. Pat. Nos. 4,498,048 and 4,580,098 show a
magnetic field generator consisting of a plurality of rings,
wherein a plurality of anisotropic permanent magnets are arranged
annularly. This ring type magnetic field generator has the great
advantage that it does not need a magnetic yoke, causing the total
weight to be correspondingly lighter.
Magnetic field generators used in MRI are required to offer an
extremely uniform and wide magnetic field space. With a central
field of 1,000 gausses, for example, the required uniformity within
a uniform field space is 100 ppm or less, and with 3,000 gausses,
30 ppm or less. However, in general, a magnetic field generating
apparatus does not provide a sufficient field uniformity without
field regulation after assembling the apparatus. Several factors
are considered responsible for the non-uniformity of the generated
field, including the working precision of parts and the assembly
precision of apparatus. Especially, in the permanent magnet type
magnetic field generator, the non-uniformity of magnetic
characteristics found in used permanent magnet blocks is a dominant
factor.
Of particular note, the aforesaid ring type magnetic field
generator, despite the advantage of not requiring the use of a pole
piece, has the problem that when compared with the yoke type, the
effect of variation in the magnetic characteristics of permanent
magnet blocks is reflected directly on the non-uniformity of field.
That is, since small magnet bricks are built into these permanent
magnet blocks, it is unavoidable with the present fabrication
technique of magnet bricks that Br (residual magnetization) should
vary considerably from one magnet brick to another, and that the
magnetizing direction should vary with the grinding and bonding
processes used in the fabrication of magnet bricks. Therefore, Br
and the magnetizing direction inevitably become different within
and between permanent magnet blocks, thus easily causing the
uniformity of field to be insufficient for use in MRI.
For these reasons, it becomes necessary to provide magnetic field
generators with a technique of regulation for improving the
uniformity of field. For example, field regulation is carried out
as follows. First, the field is measured at numerous points on the
surface of a required field space (a sphere, for example), the
non-uniformity of that field is approximated to a harmonic
function, and is then expressed as the assembly of a number of
harmonics. The uniformity of field is improved by diminishing these
harmonics by means of regulation.
Regulation is performed by mechanically moving the coil or
permanent magnet blocks which generate the main magnetic field, or
magnetic parts such as magnetic yoke.
For a magnetic field generator composed of annular or ring-shaped
magnets wherein a plurality of anisotropic permanent magnet blocks
are arranged annularly, two techniques of field regulation are
described also in the aforementioned specifications of U.S. Pat.
Nos. 4,498,048 and 4,580,098. That is, both a structure for moving
the rings along on an axial center line, and a structure for moving
the permanent magnet blocks which constitute the rings,
diametrically relative to the rings is provided.
According to the present inventors' study results when these means
were used, however, the improvement of field uniformity was found
inadequate. The reason probably is that these means alone, though
capable of reducing relatively mild low-dimensional harmonics to a
certain extent, are not effective in sufficiently reducing such
high-dimensional harmonics which arise from, for example, the
variation of magnetic characteristics in the individual magnet
bricks which constitute permanent magnet blocks.
In addition, the specification of PCT/US83/01175 (International
Publication No. WO84/00611) shows a yoke type magnetic field
generator that uses permanent magnets, and describes a scheme of
field regulation using numerous moving magnetic material screws
provided on a pole piece.
This technique of field regulation employing moving magnetic
material screws is interesting in that it can reduce
high-dimensional harmonics. However, using this means of field
regulation by moving magnetic material screws in the aforesaid ring
type magnetic field generators involves two problems:
(1) When the size of magnetic material screws is small enough
compared with the required size of a uniform field space, it is
possible to produce high-dimensional harmonics, but means for
moving small-sized magnetic material screws can only correct slight
non-uniformities, therefore they are inadequate to improve the
uniformity of ring type field generators which may generate a large
amount of high-dimensional harmonics.
(2) Generally, materials like iron with high-permeability are used
for magnetic material screws, but calculating the field generated
by a system containing materials such as iron whose magnetic
susceptibility varies markedly with the surrounding field entails
the use of repeated correction methods such as the finite element
method. Calculation in three-dimensional spaces therefore requires
a vast amount of time, and it is difficult to optimize the location
of magnetic material screws by calculation. In practice, therefore,
optimization is reached through repetition of trial and error while
moving the location of magnetic material screws. In such a method,
however, regulation is a time-consuming process and, moreover, it
is hard to improve uniformity satisfactorily.
Alternatively, another possibility is the use of a shim coil
(ordinary conducting coil for field regulation). Conventionally,
ten or more kinds of shim coils have been installed, but even that
is not satisfactory in sufficiently reducing high-dimensional
harmonics. However, installing different kinds of shim coils is
undesirable for a number of reasons, such as higher apparatus cost,
power consumption problems, and narrower internal space.
SUMMARY OF THE INVENTION
As mentioned previously, magnetic field generators used in MRI are
required an extremely uniform and wide magnetic field space, but it
is difficult to obtain a sufficient field uniformity with
conventional magnetic field generators, especially using permanent
magnets.
Therefore, the object of the present invention is to offer a
magnetic field generator with means of field regulation which can
sufficiently reduce not only low-dimensional harmonics but also
high-dimensional harmonics, and which are so arranged as to permit
easy regulation in a short time.
In the first aspect of the present invention a uniform magnetic
field generator comprises:
means for generating static magnetic field;
at least one permanent magnet piece for regulating the uniformity
of the static magnetic field; and
means for holding the permanent magnet piece or pieces.
There are multiple permanent magnet pieces and the residual
magnetization of each of the permanent magnet pieces is
substantially identical to each other.
The permanent magnet pieces do not have to be uniform in size. The
size of each of the permanent magnet pieces may be substantially
identical to each other. The permanent magnet pieces do not have to
be uniform in residual magnetization, and they can be fitted to and
removed from the means for holding the permanent magnet pieces.
The means for generating static magnetic field may be a permanent
magnet and may include a magnetic yoke. The means for generating
static magnetic field may be an electromagnet and may be a
superconducting magnet. The permanent magnet may be an annular
magnet composed of permanent magnet blocks arranged annularly.
The permanent magnet blocks may be continuous bodies and may have
at least three different sectional areas in a section vertical to
the axial direction of the annulus within the same block. The
plurality of permanent magnet blocks may be of face-symmetrical
shape.
The section of the permanent magnet blocks cut vertical to the axis
of the annular magnet may be greater at the two end portions, right
and left, than at the center main portion, and between the center
main portion and the two end potions, the block may have narrow
portions with a smaller sectional area than that of the center main
portion and the two end portions.
Each of the permanent magnet blocks may be movable in two axial
directions squarely crossing the axial direction of the annular
magnet, and the permanent magnet blocks may be rotatable around the
two axial directions.
In the second aspect of the present invention, a uniform magnetic
field generator comprises:
means for generating static magnetic field;
permanent magnet pieces for regulating the static magnetic
field;
holding members for holding the permanent magnet pieces;
a tube for fixing the holding members, the tube being inserted into
the means for generating static magnetic field;
rollers provided on the holding members and for moving the
tube;
a tube support for fixing the tube to the means for generating
static magnetic field;
rail means for bringing the tube into or out from the means for
generating static magnetic field; and
rotator means for lifting up and rotating the tube at the rail
means.
The above and other objects, effects, features and advantages of
the present invention will become more apparent from the following
description of embodiments thereof taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are front and perspective views respectively, showing
an embodiment of the uniform magnetic field generator according to
the present invention;
FIGS. 3A, 3B, 3C and 3D are partial front views showing an example
of arrangement of field regulating permanent magnet pieces;
FIG. 4 is a flowchart showing a design example of field-regulating
permanent magnet pieces;
FIG. 5 is a sectional view showing an example of securing
field-regulating permanent magnet pieces to the hold plates;
FIG. 6 is a perspective view showing an example of concrete
arrangement for field-regulating permanent magnet pieces;
FIG. 7 is a side view showing an example of permanent magnet blocks
constituting a permanent magnet unit used in the present
invention;
FIGS. 8A, 8B and 8C are sectional views through lines A--A', B--B'
and C--C' respectively of the permanent magnet block shown in FIG.
7;
FIG. 9 is a field distribution view of the permanent magnet block
shown in FIG. 7;
FIG. 10 is a perspective view showing an example of ring magnet in
one embodiment of the present invention;
FIG. 11 is a view showing the magnetizing direction of permanent
magnet blocks;
FIGS. 12, and 13 are a plan and side view respectively, showing an
example of installing the permanent magnet block shown in FIG. 2;
and
FIGS. 14A, 14B and 14C are views explaining a tube introduction
into and removal from a ring magnet, FIG. 14A being a side view,
FIGS. 14B and 14C front views.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention is explained in detail below, referring to
the drawings.
FIGS. 1 and 2 show an embodiment of the uniform magnetic field
generator according to the present invention. In FIG. 2 the
magnetic field generator and stand 10 are partially cut away. Here,
eight permanent magnet blocks 2 are arranged annularly to
constitute an annular magnet 1. The permanent magnet blocks 2 are
of trapezoidal column shape, hence the annular magnet 1 forms a
hollow trapezoidal tube. In this embodiment, the number of rings or
annuli is one, and back plates 11 of non-magnetic material are
connected to permanent magnet blocks 2. The eight permanent magnet
blocks 2, secured to a stand 10 by an adjusting jig 12, 12', form
the annular magnet 1.
Hold plates 3 of non-magnetic material are arranged annularly like
the permanent magnet blocks 2 inside the annulus of annular magnet
1. These hold plates 3 are fitted with field-regulating permanent
magnet pieces 4.
Tube 6 is arranged coaxially inside the annular magnet 1. Tube 6 is
intended to be fitted with various types of coils (RF coil,
gradient coil, etc.) for the formation of images when the present
magnetic field generator is used for MRI. Tube 6 is fixed firmly to
tube supports 7 provided at the front and rear of annular magnet 1
by tube fasteners 8, 8' of support 7. In this embodiment, the hold
plates 3 are secured to the tube 6 by mounting seats 13 and bolts
13A. The tube 6 may be drawn out of the annular magnet 1 by
removing the tube fasteners 8, 8' and utilizing rollers 9 mounted
on the plates 3. This enables the permanent magnet pieces 4 to be
mounted or replaced very easily.
The magnetizing direction of permanent magnet pieces 4 is set
parallel to the magnetizing direction 5 of permanent magnet blocks
2 fitted with hold plates 3 holding the permanent magnet pieces
4.
However, the magnetizing direction of permanent magnet pieces 4
need not necessarily be parallel to the magnetizing direction 5; it
may, of course, be set in a predetermined optional and desired
direction.
In this embodiment, permanent magnet pieces 4 are arranged on the
surface of hold plates 3 which are radially inside the annular
magnet 1, but such pieces 4 may be arranged on the surface of hold
plates 3 radially outside the annular magnet, or on the surface of
permanent magnet blocks 2, or elsewhere. However, it is preferable
to arrange them inside the annuli since that permits the size of
permanent magnet pieces to be small and additionally provides a
greater ability to correct high-dimensional harmonics.
The field-regulating permanent magnet pieces 4 will now be
explained in detail.
FIGS. 3A, 3B, 3C and 3D are views showing various examples of
arrangement for permanent magnet pieces 4, in all of which the
pieces 4 are fitted to hold plates 3 through seats 14. The
permanent magnet pieces 4 are of several sizes with their
magnetizing directions 15 predetermined for each of the hold plates
3. The magnetizing directions 15 of the permanent magnet pieces are
ordinarily parallel to the magnetizing direction of each permanent
magnet block to which each hold plate corresponds. The magnetizing
direction of each of the permanent magnet pieces on each hold plate
3 is same/opposite. It is preferable that there are some permanent
magnet pieces having an opposite magnetizing direction to that of
other permanent magnet pieces, because the maximum size of the
permanent magnet pieces can be reduced. The permanent magnet pieces
4 may be different from each other in size.
In another embodiment of the present invention, the size and
mounting position of permanent magnet pieces 4 may be prefixed so
as to use permanent magnet pieces 4 of different residual
magnetization and alternatively the size of permanent magnet pieces
4 may be prefixed so as to increase the flexibility of their
mounting position. The permanent magnet pieces 4 may be of several
residual magnetization or may be of individually different residual
magnetization.
The magnet materials available for these permanent magnet pieces 4
include Sm-Co base, Nd-Fe-B base, and other rare earth magnet
material, ferrite magnet material, or their analogues. The same is
true for the case of permanent magnet blocks 2.
FIG. 4 is a flowchart showing an example of procedure to determine
the optimum arrangement of field regulating permanent magnet pieces
4. Several kinds of magnet pieces 4 are prepared which are equal in
residual magnetization and different in size. The design
information to size magnet pieces 4 should be fixed as calculated,
for example, as follows.
First, the turbulence of the field is evaluated. The turbulence of
a field of view (be a sphere here) is known to be fully assessable
by looking into the turbulence of the surface of sphere. At
numerous points (n points) on the surface, the magnetic field is
measured (H.sub.1, H.sub.2, H.sub.3 . . . H.sub.n) in the main
magnetid field direction.
C.sub.p, the turbulence of a certain point P, is defined by the
following equation:
where, H.sub.0 is the field at the center of sphere. C.sub.p, the
turbulence of point P, is a value having plus and minus signs, and
field turbulence is expressed by C at n number of points.
Next, regulation information must be obtained correspondingly to
this field turbulence. Various positions (here predetermined
positions on hold plates 3 where magnet pieces 4 may be mounted)
are marked with a serial number, and the size of magnet pieces 4 is
given as .DELTA.R.sub.i with respect to the i-th position. The
magnetizing direction 15 of permanent magnet pieces 4 is determined
relative to the permanent magnet blocks 2 to which the hold plates
3 are opposite, and assigned a plus or minus value.
Against a given turbulance, permanent magnet pieces 4 with various
sizes are arranged and fixed at various positions. If the sizes of
the permanent magnet pieces 4 are sequenced in this serial number
order, they become vectors of (.DELTA.R.sub.1, .DELTA.R.sub.2,
.DELTA.R.sub.3, . . . .DELTA.R.sub.m). These vectors are called
regulating vectors or shimming vectors.
The shimming vector, .DELTA.R, must be sought in order to carry out
regulation of field. As a basic equation, the following is
considered: ##EQU1## n: total number of coefficients of turbulence
development m: total number of regulating mechanisms
Here, C is a matrix vector of "field turbulence" obtained from
field measurements, and G is a matrix of (n.times.m) indicating how
the field of various measuring points changes relative to
microchanges in the size of permanent magnet pieces 4 at various
positions. G is called a sensitivity matrix.
.DELTA.R, an m-th matrix vector indicating the amount of parameter
change, that is, the size of permanent magnet pieces 4 fixed at
various positions, is found as a solution to Equation (2). This
means that a new turbulence (-C) to nullify the field turbulence C
is created.
Equation (2) is solved in the following manner. G, being a
rectangular matrix of (n.times.m), can be converted with a set of
orthogonal transformations, S and T, into the following forms
(resolution into specific values):
Hence
-C=SDT.sup.+ .multidot..DELTA.R (4)
Writing this as
Equation (2) will have been transformed thus ##EQU2##
The various elements of vector .DELTA.R are found as:
If the matrix having (1/d.sub.i) as a diagonal element is given as
D.sup.-1, ##EQU3##
That is, the portion of (TD.sup.-1 S.sup.+) has become G.sup.+, an
inverse matrix of G. Once G.sup.+ is thus found, .DELTA.R, a
shimming vector which nullifies the turbulence of a certain field,
is immediately found.
The concrete procedure is shown in according with FIG. 4.
First, the positions where magnet pieces 4 can be mounted are
determined. These positions are taken, for example, to be lattice
points defined at regular intervals on hold plates 3. Then, obtain
by computing the information about field changes at the various
measuring points on the surface of sphere at the time when a unit
amount of magnet pieces have been arranged at the various
positions, and this is prepared in the form of G matrix. Next,
G.sup.+, the generalized inverse matrix, is derived from the G
matrix in the manner mentioned before. After assembly of the
apparatus, an NMR probe is used to precisely measure the field at
various measuring points (n) on the surface of sphere, and find the
field turbulence C.
The difference between the greatest and least values of field
uniformity, as derived from the results of these field
measurements, is divided by the absolute magnetic field (mean
value), and the result expressed in terms of ppm. If this value is
not up to desired value (which, for example, is required to be 30
ppm or under in a unit having a center field of 3,000 gausses),
Equation (17) is used to derive the size (shimming vector) .DELTA.R
of magnets arranged at various positions from G.sup.+ and C.
Permanent magnet pieces 4 are prepared in accordance with the data
on permanent magnet pieces 4, including position, size, magnetizing
direction, and residual magnetization dependent on magnet materials
used, which have been determined through the above-mentioned
procedure. These pieces 4 are arranged and fixed on hold plates 3
via seats 14. They need to be fixed in place securely and firmly
since their displacement after mounting detracts from the field
uniformity. Regulation work is concluded by arranging and fixing
the permanent magnet pieces 4. The above operation is repeated if
further improvement of uniformity is desired. Errors arising from
the variation of residual magnetization, movement of mounting
position, etc., of permanent magnet pieces 4 can be also be
approximated to zero by repeating regulation.
Permanent magnet pieces 4, connected onto seats 14 of non-magnetic
material as shown in FIG. 5, are fixed to hold plates 3 of
non-magnetic material securely by screws 16 also of non-magnetic
material.
FIG. 6 shows an example of a preferred arrangement of
field-regulating permanent magnet pieces 4 on a hold plate 3.
Numerous open mounting holes 17 are located at predetermined
positions on hold plates 3. Permanent magnet pieces 4 and seats 14
are fixed into desired mounting holes 17 on hold plates 3 by screws
16, then hold plates 3 are fixed to tube 6, matching them with
permanent magnet blocks 2.
Next, the shape of permanent magnet blocks 2 in this embodiment
will be explained in detail.
FIG. 7 is a side view showing an example of permanent magnet block
2 in the magnetic filed generator of the present invention.
FIG. 8A is a sectional view of permanent magnet block 2 cut along
line A--A' vertical to the annular axis (shown by an arrow Z) of
FIG. 7.
FIG. 8B is a sectional view of permanent magnet block 2 cut along
line B--B' vertical to the annular axis of FIG. 7.
FIG. 8C is a sectional view of permanent magnet block 2 cut along
line C--C' vertical to the annular axis of FIG. 7.
Here, permanent magnet block 2 is composed of main portion 18
having a sectional area corresponding to the required filed
intensity, narrow portions 19 of small sectional area connected to
both ends of this main portion 18, and tip portions 20 of greater
sectional area than that of main portion 18.
In this permanent magnet block 2, the thickness h.sub.3 and length
l.sub.3 of central main portion 18 are determined by the following
equations correspondingly to required field intensity H.sub.0 :
##EQU4## where R is the required diameter of bore and Br the
residual magnetization of the permanent magnet used. C.sub.1 to
C.sub.3 are coefficients, and in order to obtain required field
intensity, it is desirable to set C.sub.1 =0.57 to 0.62, C.sub.2
=0.81 to 1.10, and C.sub.3 =0.029 to 0.043.
The thickness h.sub.1 and l.sub.1 of tip portion 20 at both ends of
permanent magnet block 2 are determined by the following equations
relative to the thickness h.sub.3 and length l.sub.3 of the main
portion 18:
The length l.sub.2 of narrow portion 19 connected to both ends of
main portion 18 is determined by the following equation relative to
the length l.sub.3 and thickness h.sub.3 of main portion 18 and to
the thickness h.sub.1 of tip portion 20: ##EQU6## where C.sub.6 is
a coefficient, and in order to obtain required field uniformity it
is desirable to set C.sub.6 =0.9 to 2.0.
Also, the thickness h.sub.2 of narrow portion 19 is determined by
the following equation relative to the thickness of h.sub.3 of main
portion 18, the thickness of h.sub.1 of tip portion 20, and the
length l.sub.2 of narrow portion 19: ##EQU7## where C.sub.7 is a
coefficient, and in order to obtain required field intensity, it is
desirable to have C.sub.7 =54 to 72 (mm).
The above-mentioned sectional shape, that is, the area of section
cut vertical to the annular axis is larger at both tip portions 20,
right and left, than at the central main portion 18. A permanent
magnet block 2 of such shape has narrow portions 19 with smaller
sectional area than that of either the central main portion 18 or
the tip portions 20 in order to obtain a wide and uniform field.
FIG. 9 is a view corresponding to the side section of an annular
magnet unit and showing magnetic field changes along the annular
axial direction. It is apparent from FIG. 9 that uniformity has
been improved by the provision of smaller sectional areas 19
between the center portion 18 and tip portions 20.
The permanent magnet block shown in FIG. 7 has a shape free from
irregularities on the inside, but the present invention is not so
limited. However, the permanent magnet block usually is magnetized
while in the form of a smaller semi-block and then assembled, but
the absence of irregularities on the surface forming the inside is
preferable since that permits easy positioning or the like during
assembly and assures highly accurate assembly.
Also, the permanent magnet block of FIG. 7 is of symmetrical shape,
but this need not be a limiting factor if the aforementioned
conditions are met.
The permanent magnet material usable for permanent magnet block 2
is an oriented anisotropical permanent magnet, and it is
particularly desirable that it has a large Br (residual
magnetization) value and a large coercive force.
Materials usable for this purpose, for example, are the rare
earth-cobalt base magnet such as SmCo.sub.5, Sm.sub.2 Co.sub.17, or
the like, the rare earth-iron base magnet, such as Nd-Fe-B, or the
like, and ferrite magnet, or their analogues. Among these, magnets
of high maximum energy product BHmax and low specific gravity are
preferable. These types of magnets may include permanent magnets
which are composed principally of 8 to 30 atomic percent Ln (Ln
represents at least one of the Y-bearing rare earths), 2 to 28
atomic percent B, and 42 to 90 atomic percent Fe, and whose
principal phase is tetragonal.
FIG. 10 is a perspective view of annular magnet 1 composed of
permanent magnet blocks 2 shown in FIG. 7. This annular magnet 1 is
formed by an annular arrangement of permanent magnet blocks 2.
In FIG. 10, the permanent magnet unit based on one annular magnet 1
consists of eight anisotropic permanent magnet blocks 2, but
annular magnet 1 may also be formed by more or fewer blocks 2.
According to the inventors' study results, the greater the number
of permanent magnet blocks 2, the lower the amount of magnet
necessary to obtain a concentric field. Also, when the number of
blocks 2 forming the annular magnet 1 is even, a uniform field is
easy to obtain since annular magnet 1 is highly symmetric.
In FIG. 10, moreover, annular magnet 1 is of regular octagonal
shape, but the magnet need not be a regular polygon provided the
opposed blocks are substantially equal in shape. It may
alternatively be horizontally long or vertically long.
FIG. 11 is a view showing the magnetizing direction 21 of permanent
magnet block 2. If the axial direction of annular magnet 1 vertical
to the paper surface is given as Z, and the plane vertical to the
axis of annular magnet 1 as XY plane, then the permanent magnet
blocks 2 forming the annular magnet 1 should desirably be ones
arranged annularly so that the orientation of the easy axis of
magnetization may become an angle determined in the following
equation:
where .theta. is the angle between the radially symmetrical line 22
of permanent magnet block 2 and the X axis, and o the angle formed
by the easy axis of magnetization (magnetizing direction) 21 of
block 2 and line 23 parallel to the X axis.
In the present invention, furthermore, means of regulation by
permanent magnet pieces 4 may be used in combination with other
means of adjustment, for example, with movement of permanent magnet
blocks 2 constituting the annular magnet 1, or with shim coils.
The embodiment shown in FIG. 1 may be equipped with means of
adjustment by movement of permanent magnet blocks 2.
Connected to permanent magnet blocks 2 are back plates 11, of
non-magnetic material, on the face corresponding to the annular
outside when the blocks 2 are connected to form the annular magnet.
On the other hand, adjusting jigs 12 and 12' for moving permanent
magnet blocks 2 and adjusting the field, are fixed to annular stand
10 onto which annular magnet 1 can be loose fitted. These adjusting
jigs 12 and 12' are fitted to back plates 11 to support each of
permanent magnet blocks 2 movably. Adjusting jigs 12 and 12' are
designed to move the position of each permanent magnet block 2
after assembly of the permanent magnet unit and improve field
uniformity. These adjusting jigs 12 and 12' and the stand 10 are
composed of non-magnetic material.
Next, preferred examples of such adjusting jigs 12 and 12' are
detailed in FIGS. 12 and 13.
FIG. 12 is a plan view of the permanent magnet unit of FIG. 2 and
shows an example of fitting each of permanent magnet blocks 2 to
stand 10. FIG. 13 is a side view of the permanent magnet block
shown in FIG. 12 as viewed from the side indicated by an arrow in
FIG. 12.
Permanent magnet block 2 has back plate 11 connected to it as
mentioned above, and the permanent magnet block 2 is fixed to stand
10 in a longitudinal direction at two points, right and left, by
adjusting jigs 12 and 12'. Adjusting jigs 12 and 12' are designed
to permit adjustment of the position and posture of the permanent
magnet blocks 2, and they are arranged in order to permit their
adjustment in the X and Y directions as well as rotation around the
X and Y axes.
More particularly, in FIGS. 12 and 13, guide members 30 hold slide
shafts 28 so that they are rotatable and movable up and down. The
guide members 30 are fixed to back plates 11 located almost
symmetrically to the center face of stand 10. Bearing members 26
and 27 are fixed on the side of stand 10. These bearing members 26
and 27 support rotating shafts 32 and 31 respectively. In addition,
in FIGS. 12 and 13, swing arm 33 is supported rotatably on the
rotating shaft 31 of jig 12' and, the end of arm 33 is connected to
slide shaft 28 via pin 34. As a result, some play will be provided
between swing arm 33 and pin 34.
The rotating shaft 32 of jig 12 is connected directly to slide
shaft 28. Machine bolts 35 are screwed into threaded holes provided
respectively on the arms 26A and 27A of bearing members 26 and 27.
They are screwed into place inward respectively past the opposed
arms. In FIGS. 12 and 13, jig 12' on the left and jig 12 on the
right directly support swing arm 33 and slide shaft 28
respectively. Push-pull bolts 36 are fitted into slide shafts 28.
Screwing in the bolts 36 or screwing them back enables slide shafts
28, themselves to be moved up or down.
In adjusting jigs 12 and 12' arranged thusly, to move them in the X
direction, for example, the two push-pull bolts 26 are turned the
same amount, right and left, in the same direction. To move the
jigs 12 and 12' in the Y direction, of the four machine bolts 35
provided on the right and left bearing members 26 and 27 as shown
in FIG. 12, the two machine bolts 35 on the desired side (in the
figure, either upper or lower bolts) can be loosened
correspondingly to the amount of movement and the machine bolts 35
on the opposite side likewise tightened up.
Also, rotation around the X axis can be effected by moving the
machine bolt 35 to the right and left in opposite directions and by
this operation slide shafts 28 may be rotated within guide members
30.
Rotation around the Y axis can be effected by moving the push-pull
bolts 36, used during movement in the X direction, in opposite
directions right and left. Rotation at this time involves rotating
shafts 31 and 32 as well as pin 34. In addition, changes in the
distance between right and left adjusting jigs 12' and 12 arising
from Y axis rotation are absorbed by swing arms 33.
Movement of tube 6 into and out of annular magnet 1 will now be
described with reference to FIGS. 14A, 14B and 14C. FIG. 14A is a
side view of tube car 40 used for introduction and removal of tube
6 and of the embodiment of magnetic field generator shown in FIG.
1, with partial cutoffs shown of permanent block 2, stand 10, tube
support 7, and tube car 40. FIGS. 14B and 14C are front views taken
from the side of tube car 40. When the tube 6 is taken out of
annular magnet 1, tube fasteners 8 and 8' are first removed, and
tube 6 is lowered on permanent magnet blocks 2. At this time, tube
6 is in contact with permanent magnet blocks 2 via rollers 9
mounted on hold plates 3.
The tube car 40 may then be moved to the position shown in FIG.
14A. The face of racks 41 of tube car 40 is in line with the face
of permanent magnet blocks 2 with which the rollers 9 are in
contact. Tube 6 can then be moved smoothly onto tube car 40 by
rollers 9. This operation is reversed when putting tube 6 into
annular ring 1. Tube 6 after it has been moved onto tube car 40 is
shown in FIG. 14B. Tube 6 may be made rotatable by four tube
rotating jigs 42 attached to tube car 40. That is, each roller 44
may be raised by rotating the bolt 43 of each tube rotating jig 42,
and thus tube 6 supported by these rollers 44 is also raised.
Consequently, tube 6 may be rotated freely on car 40. This makes it
possible to fit hold plates 3 to tube 6 or remove them therefrom
very easily. As a result, it is easy to fit or remove permanent
magnet pieces 4.
Next, the working and assembly procedures for various parts on the
magnetic field generator according to the present invention will be
described.
(1) Bricks
Permanent magnet blocks 2 are assembled from small anisotropic
permanent magnet bricks, for example, about 20 mm.times.30
mm.times.50 mm.
(2) Forming Into Semi-blocks
Semi-blocks are made by splitting a permanent magnet block into
several parts, and their size is limited by magnetizer capacity,
ease of handling, etc. They are made to be, for example, 200
mm.times.200 mm.times.200 mm in size. These unmagnetized
anisotropic permanent magnet bricks are bonded together with easy
axes of magnetization aligned, or they are precut into required
shapes before bonding together. Following this, they are made into
semi-blocks of required shape and magnetizing direction by
repeating the cut and bond cycle. Moreover, these semi-blocks are
completely ground and finished to required dimensions and
tolerances.
(3) Magnetizing
The semi-blocks constructed in the above manner are magnetized
according to the magnetizing direction with a required magnetic
field.
(4) Forming Into Blocks
The magnetized semi-blocks are bonded to close tolerances one after
another by the use of jigs until permanent magnet blocks such as
shown in FIG. 2 are obtained.
(5) Forming Into Unit (Annulus)
A plurality of permanent magnet blocks 2 thus constructed are
mounted in sequence on stand 10 by the use of jigs 12 and 12' to
complete a permanent magnet unit.
(6) Initial Setting
The position of permanent magnet blocks 2 is adjusted by the use of
adjusting jigs 12 and 12' and set to design values.
Next, magnetic field regulation procedure is explained.
(1) Field Measurement
An NMR probe is used to measure the magnetic field with respect to
numerous points (for example, 91 points) on the surface of a field
of view (sphere) of required size at the center of the permanent
magnet unit.
(2) Evaluation of Uniformity
The difference between the greatest and least values of magnetic
field uniformity, as derived from the above field measurement
results, is divided by the absolute field (means value) and
expressed in terms of ppm.
(3) Regulation
As mentioned previously, the magnetic field turbulence is evaluated
through the field measurement of paragraph (1), a regulating vector
is worked out by computation, the size and layout of permanent
magnet pieces 4 are determined, and they are then mounted and
fixed. Prior to this regulation by permanent magnet pieces 4,
however, adjustment by the movement of permanent magnet blocks 2 is
carried out as the occasion demands. The procedure is basically the
same as that for regulation by permanent magnet pieces 4. That is,
with the amount of movement of each permanent magnet block 2 as a
parameter, a regulating vector which will improve uniformity is
worked out by computation. Using the adjusting jigs 12 and 12',
each permanent magnet block is moved to specified position.
As an example of a magnetic field generator, an annular magnet type
generator composed of anistropic magnets arranged annularly has
been explained in detail But the present invention is not limited
to above-mentioned example.
The present invention can be applied to other permanent magnet
types, for example, a permanent magnet with pole pieces and a
magnetic yoke, generators or electromagnet type magnetic field
generators. For example, the present invention can be applied to an
electromagnet type generator utilizing double Helmholtz coils which
are generally used for generating a static magnetic field in
electromagnets. Double Helmholtz coils are composed of
symmetrically arranged four air-coils, and these coils are fixed to
a stand. Permanent magnet pieces are arranged and fixed on a hold
plate and this hold plate is fixed to a tube, the tube is then
inserted into coils. For split coils, such as double Helmholtz
coils, the tube may be arranged in coaxial relation to the axis of
coils or may be arranged between coils perpendicular to the axis of
coils. The tube may be supported by a tube support in the same
manner as the annular magnet type permanent magnet. A coil for an
electromagnet may be a super-conducting coil.
As has been stated above, the uniform magnetic field generator of
the present invention provides for the arrangement of permanent
magnet pieces, so even extremely small permanent magnet pieces are
fully capable of uniformity improvement. Also, the magnetic field
distribution formed in space by permanent magnet pieces can be
precisely calculated in a short time for each permanent magnet
piece, and thus an optimum arrangement for uniformity improvement
is easily obtained. Therefore, according to the present invention,
sufficient adjustment can be provided not only for magnetic field
non-uniformity due to errors in the shape or position of permanent
magnet block or coils, but also for errors including, for example,
high-order harmonics arising from the variation of magnetic
characteristics in the magnet bricks which constitute the permanent
magnet blocks. Therefore, an extremely uniform and wide magnetic
field space is obtained. Moreover, this generator has another major
advantage in that since it permits easy mounting and fixing of
permanent magnet pieces, regulation is available accurately and
quickly. This magnetic field generator is preferable for use in
such equipment as MRI.
* * * * *